1. Field of the Invention
The present invention relates to an electroconductive roller having an electroconductive cellular material layer comprised of a polyurethane composition. Particularly, the present invention relates to an electroconductive roller having a small diameter in foamed cells thereof, a small variation width in the diameter in the foamed cells thereof, a small variation width in the hardness thereof, and a small variation width in the electric resistance thereof. More particularly, the present invention relates to a roller whose electroconductive cellular material layer is composed of a thermosetting one-part polyurethane composition with which a gas is mechanically mixed, a method of manufacturing the electroconductive roller, and an image-processing apparatus having the electroconductive roller.
2. Description of the Related Art
In recent years, as electrophotographic technique progresses and an image-processing apparatus using the electrophotographic technique is provided with technique of forming images in color at a high speed, a semi-electroconductive member utilized in an electrophotographic process is demanded to have a high-level technique. An elastic roller utilized for transfer, development, toner supply, and electric charging is demanded to be semi-electroconductive and have a uniform electric resistance to obtain a high-quality image. The elastic roller is also demanded to have a smaller volume and a high process efficiency. In addition, the elastic roller is demanded to be soft and have a uniform hardness to prevent other members such as an electrophotographic photoreceptor of the image-processing apparatus from being damaged. A soft elastic roller allows a large nip width to be formed between it and the electrophotographic photoreceptor and the like when the soft elastic roller is pressed against the electrophotographic photoreceptor. Thus a higher process efficiency can be obtained. Further the soft elastic roller hardly damages members of the electrophotographic photoreceptor, when the soft elastic roller is pressed against the electrophotographic photoreceptor and the like. Moreover the elastic roller is demanded to have a uniform hardness to obtain a uniform high-quality image.
In a conventional method, to make the elastic roller have a low hardness and a large nip width, process oil and a softening agent such as ester plasticizer are added to a rubber composition or a polyurethane composition.
But this conventional method of making the elastic roller have a low hardness has a problem that the softening agent causes bleeding and migration contamination to occur. Thereby the electrophotographic photoreceptor and the like are deteriorated, which causes generation of defective images.
To solve this problem, a method of using an electroconductive blowing agent is adopted to make the elastic roller have a low hardness. In this method, a chemical blowing agent such as azodicarbonamide (ADCA) or 4,4-oxybis (benzene sulfonyl hydrazide) (OBSH) is added to a rubber composition. Owing to heat applied thereto during vulcanization, a gas is generated from the chemical blowing agent. Various improvements have been made for the kind of a blowing agent and an assistant blowing agent, a vulcanizing condition, a vulcanizing agent, and a vulcanizing accelerator.
For example, in Japanese Patent Application Laid-Open No. 2003-105119 (patent document 1), the present applicant proposed a rubber roller having an electroconductive layer containing epichlorohydrin rubber and a chemical blowing agent of OBSH. The rubber roller having the electroconductive layer has a considerably large variation width of uniformity in foamed cell diameters in the range of 10 μm to 100 μm.
The rubber roller having the electroconductive layer described in the patent document 1 is uniform in the electric resistance and has a superior performance in the present technique level. But epichlorohydrin rubber containing chlorine is used for the rubber roller. Hence when the rubber roller is destroyed by fire, there is a possibility that hydrogen chloride gas is generated and that dioxine is generated in dependence on a burning condition. Thus it is difficult to destroy the rubber roller, and hence the disposal cost is high.
Such being the case, to reduce the cost required for the disposal and prevent environmental contamination, in recent years, there is a tendency for polyurethane to be used instead of the epichlorohydrin rubber for the electroconductive cellular material layer of the elastic roller. The use of the polyurethane allows production of the elastic roller not containing chlorine or bromine and the molecular structure of the composition of the electroconductive cellular material layer to be designed considerably freely by arbitrarily altering the structure of polyol and isocyanate compound and the mixing ratio thereof. Hence the use of the polyurethane makes it easy to adapt with various users' demands.
As shown in the non-patent document 1, several methods are used to foam a polyurethane composition. For example, in the method (by physical blowing agent) of obtaining a foam disclosed in Patent Publication No. 2577884 (patent document 2), a flon-containing solvent is used as the blowing agent, and the blowing agent is vaporized by reaction heat generated when the polyurethane is formed by reaction between polyisocyanate and polyol. But this method has a problem that progress speed of the reaction is different in dependence on portions of a mold in a polyurethane reaction. Consequently there is a variation in foamed states in dependence on portions of the mold.
A foaming method that is carried out by using water is disclosed in Japanese Patent Application Laid-Open No. 2001-341139 (patent document 3). A prepolymer process is used in this method. Thus this method allows a curing reaction to progress, but there is a variation in the speed of the reaction between water and a prepolymer in dependence on portions of a mold. Consequently there is a variation in the foamed state.
The polyurethane foam proposed in Patent Publication No. 3278316 (patent document 4) is formed by foaming polyol which has reacted with polyisocyanate by using a mechanical froth method (method of mixing foamable cells into polyol). In the method disclosed in the patent document 4, foamable cells are generated by mixing air into the polyurethane composition with a mixer to obtain a foam. In this method, it is difficult to mix foamable cells uniformly into the polyurethane composition. Thus as in the case of the above-described two methods, there is a large variation in the foamed state and the foamed cell diameter. Hence the method disclosed in the patent document 4 is incapable of producing products that withstand practical use.
To use the elastic roller for the image-processing apparatus, it is necessary to impart electroconductivity to the cellular material layer of the elastic roller to provide the elastic roller with semi-electroconductivity and reduce a variation in the electric resistance thereof to obtain preferable images.
To impart electroconductivity to the cellular material layer, the following three methods are known: In one method, an electroconductive filler such as powder of metal oxides and carbon black is added to a rubber composition or the like to make the cellular material layer electroconductive. In another method, ionic-electroconductive rubber/elastomer or an ionic-electroconductive agent is added to the rubber composition or the like. In still another method, both the electroconductive filler and the ionic-electroconductive rubber/elastomer or the ionic-electroconductive agent are added to the rubber composition or the like.
When the cellular material layer is made electroconductive, the electric resistance of a semielectroconductive region changes suddenly owing to a slight change of an addition amount of the electroconductive filler, although the electric resistance of the semielectroconductive region is demanded to have a constant electric resistance. Thus it is very difficult to control the electric resistance. Another problem of this method is that it is very difficult to disperse the electroconductive filler uniformly in the composition. Therefore the electric resistance of the elastic roller is liable to have variations in its circumferential and longitudinal directions. In addition, there is variations in the electric resistance of products. In particular, when the elastic roller is formed by molding materials in a small mold having a size almost equal to that of the electroconductive roller, the electroconductive filler is distributed non-uniformly. Still another problem of this method is that the electric resistance of the electroconductive roller depends greatly on an applied voltage and hence is not constant. When carbon black is used as the electroconductive filler, this tendency is outstanding. Such a phenomenon makes a mechanical control difficult in image-processing processes such as electric charging, development, transfer, and fixing. Thus the cost of the electroconductive roller is high. When the carbon black is used as the electroconductive filler, the electroconductive roller is colored in black and hence cannot be colored in a desired color. Thus it is difficult to find that the electroconductive roller has been contaminated with toner.
It is preferable to make the cellular material layer ionic-electroconductive because the cellular material layer made electroconductive has the above-described problems. The electric resistance of an ionic-electroconductive roller practically used depends greatly on environment. Thereby it is necessary to provide an image-processing apparatus with a large power source and in addition, a complicated control mechanism. Consequently there is a possibility that the cost for manufacturing the ionic-electroconductive roller and the running cost become high. Because the electric resistance of the ionic-electroconductive roller depends greatly on environment, it is necessary to conduct many environmental tests in development. Consequently the development cost increases. There is a possibility that the ionic-electroconductive roller contaminates the electrophotographic photoreceptor in dependence on a component used therefor. In consideration of the above-described problems, improvement is required for the ionic-electroconductive roller.
Patent document 1: Japanese Patent Application Laid-Open No. 2003-105119
Patent document 2: Patent Publication No. 2577884
Patent document 3: Japanese Patent Application Laid-Open No. 2001-341139
Patent document 4: Patent Publication No. 3278316
Non-patent document 1: Page 67 to 218 of “Plastic foam handbook” published by Nikkan Kogyo Shinbun Inc. on February 28, in the 48th year of Showa.